In storage systems, as tracks and sectors are spaced closer together and as the spin rates produced by the spindle motor are increased, the likelihood of an error occurring while reading the storage media increase significantly. Storage systems employ a number of different techniques to eliminate or reduce such errors that may occur when reading data from a storage media. Most error detection and correction techniques in storage systems employ redundant information and special hardware. Error correction codes, for example, such as Low Density Parity Check (LDPC) codes, are often used to detect and correct such errors. LDPC codes are block codes based on a parity check matrix, H. See, for example, R. G. Gallagher, “Low-Density Parity-Check Code,” IEEE Trans. Inform. Theory, vol. IT-8, 21-28 (January 1962).
When an error is encountered during a read operation, a retry operation is typically implemented, whereby a signal may be re-read from a buffer or from the storage media itself (or both). When a reread operation is performed from the storage media, the operating conditions may be varied, for example, by repositioning the transducer, to obtain a different signal. The reread operations typically continue until the erroneous data is correctly read from the storage media or until a predefined number of retry operations have been performed.
In one existing retry technique, an “on-the-fly” soft channel detector performs the initial read operation. If the “on-the-fly” detector encounters an error, a second soft channel detector may process buffered samples to perform an electronic retry that typically employs additional iterations than the “on-the-fly” detector. If the electronic retry fails, a physical re-read operation will be initiated, whereby the transducer is repositioned over the target area to obtain a new signal.
A need exists for improved physical re-read operations in a hard disk drive. A further need exists for improved methods and apparatus for selective data retention decoding in a hard disk drive.